package com.dev.op;

import org.springframework.security.core.parameters.P;

import java.util.*;
import java.util.concurrent.locks.Condition;
import java.util.concurrent.locks.ReentrantLock;

public class main {

    List<Integer> ls = new ArrayList<>();

    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        String str = sc.nextLine();
        int n = str.length();
        String str1 = str.substring(0, n/2);
        String str2 = str.substring(n/2);

        //逐位对比即可
        int cnt = 0;
        for(int i=0;i<n/2;i++){
            //    n/2+i
            if(str1.charAt(i) != str2.charAt(i)){
                cnt++;
            }
        }
        System.out.println(cnt);

    }

}




